1. Field of the Invention
The present invention relates to an image recording apparatus and method, and more particularly to an image recording apparatus and method for determining recording defects of an image-recording element in an inkjet recording apparatus or other image recording apparatus for recording images on a printing medium by a recording head having a plurality of image-recording elements, and to a technique for compensating for recording defects thereof.
2. Description of the Related Art
Inkjet recording apparatuses have an inkjet head (print head) in which a large number of nozzles are arranged, and images are formed on recording paper by ejecting ink droplets from the nozzles while moving the print head and the paper relatively to each other. There are cases in which some of the nozzles of the large number of nozzles no longer eject ink for some reason, the amount of ink ejected (the dot size resulting from the ejection of a droplet on the recording paper) and the droplet deposition positions become defective (defective flight direction, non-uniform nozzle positions), and other ejection defects occur. The presence of such defective nozzles causes the quality of the recorded image to be degraded, thus countermeasures thereto are required.
Conventionally, known methods for determining ejection defects in nozzles include (1) a method for measuring a printed test pattern, (2) a method for measuring an actual print job (the printed result of a target image that actually requires printing output), and (3) a method for measuring the characteristics during ejection inside the head.
However, the method (1) for measuring a printed test pattern requires that a special test pattern be printed, which is separate from a target image that actually requires printing. Moreover, there are drawbacks in a simple pattern in that the results are affected by errors in the measurement positions and that it is difficult to determine defective nozzles. Furthermore, there is a drawback in that the results are affected by variability in the output of the line sensor for reading the test pattern.
In the case of the method (2) for measuring an actual print job, the actual print job, which is the measurement object, is usually an intricate image, so that there are drawbacks in that it becomes difficult to determine whether image defects are due to a defective nozzle or to the original image content, and to accurately determine a defective nozzle, due to the effect of errors in the measurement positions. Moreover, the results are affected by variability in the line sensor in the same manner as the above test pattern.
Japanese Patent Application Publication No. 5-301427 discloses an example of the above method (2) whereby the droplet deposition image data and the data to be recorded are compared, and dot deficiencies due to ejection defects are corrected. More specifically, dots on the recording paper are read with photoelectric transducing elements arranged with the same pitch as the pitch of the nozzles of the recording head to detect a non-ejection of ink; however, in this method, when the image has a high density, the sensor output difference with the surroundings is smaller, so that defective ink-droplet ejection cannot be accurately determined by comparing individual dots. In particular, in the case of high-density nozzles used for high resolution image recording, the observation area of each pixel of the line sensor is greater than the nozzle pitch, so that it becomes even more difficult to determine a defective ejection nozzle when the conveyance errors of recording paper and the like are also taken into consideration.
Moreover, the method (3) for measuring the characteristics during ejection inside the head can accurately determine a defective nozzle; however, there is a drawback in that it is difficult to ascertain the degree of deficiency.